Rubisco mutants of Chlamydomonas reinhardtii enhance photosynthetic hydrogen production.

Molecular hydrogen (H2) is an ideal fuel characterized by high enthalpy change and lack of greenhouse effects. This biofuel can be released by microalgae via reduction of protons to molecular hydrogen catalyzed by hydrogenases. The main competitor for the reducing power required by the hydrogenases is the Calvin cycle, and rubisco plays a key role therein. Engineered Chlamydomonas with reduced rubisco levels, activity and stability was used as the basis of this research effort aimed at increasing hydrogen production. Biochemical monitoring in such metabolically engineered mutant cells proceeded in Tris/acetate/phosphate culture medium with S-depletion or repletion, both under hypoxia. Photosynthetic activity, maximum photochemical efficiency, chlorophyll and protein levels were all measured. In addition, expression of rubisco, hydrogenase, D1 and Lhcb were investigated, and H2 was quantified. At the beginning of the experiments, rubisco increased followed by intense degradation. Lhcb proteins exhibited monomeric isoforms during the first 24 to 48 h, and D1 displayed sensitivity under S-depletion. Rubisco mutants exhibited a significant decrease in O2 evolution compared with the control. Although the S-depleted medium was much more suitable than its complete counterpart for H2 production, hydrogen release was observed also in sealed S-repleted cultures of rubisco mutated cells under low-moderate light conditions. In particular, the rubisco mutant Y67A accounted for 10-15-fold higher hydrogen production than the wild type under the same conditions and also displayed divergent metabolic parameters. These results indicate that rubisco is a promising target for improving hydrogen production rates in engineered microalgae.

First crystal structure of Rubisco from a green alga, Chlamydomonas reinhardtii.

The crystal structure of Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase) from the unicellular green alga Chlamydomonas reinhardtii has been determined to 1.4 A resolution. Overall, the structure shows high similarity to the previously determined structures of L8S8 Rubisco enzymes. The largest difference is found in the loop between beta strands A and B of the small subunit (betaA-betaB loop), which is longer by six amino acid residues than the corresponding region in Rubisco from Spinacia. Mutations of residues in the betaA-betaB loop have been shown to affect holoenzyme stability and catalytic properties. The information contained in the Chlamydomonas structure enables a more reliable analysis of the effect of these mutations. No electron density was observed for the last 13 residues of the small subunit, which are assumed to be disordered in the crystal. Because of the high resolution of the data, some posttranslational modifications are unambiguously apparent in the structure. These include cysteine and N-terminal methylations and proline 4-hydroxylations.

Alanine-scanning mutagenesis of the small-subunit beta A-beta B loop of chloroplast ribulose-1,5-bisphosphate carboxylase/oxygenase: substitution at Arg-71 affects thermal stability and CO2/O2 specificity.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) enzymes from different species differ with respect to carboxylation catalytic efficiency and CO2/O2 specificity, but the structural basis for these differences is not known. Whereas much is known about the chloroplast-encoded large subunit, which contains the alpha/beta-barrel active site, much less is known about the role of the nuclear-encoded small subunit in Rubisco structure and function. In particular, a loop between beta-strands A and B contains 21 or more residues in plants and green algae, but only 10 residues in prokaryotes and nongreen algae. To determine the significance of these additional residues, a mutant of the green alga Chlamydomonas reinhardtii, which lacks both small-subunit genes, was used as a host for transformation with directed-mutant genes. Although previous studies had indicated that the betaA-betaB loop was essential for holoenzyme assembly, Ala substitutions at residues conserved among land plants and algae (Arg-59, Tyr-67, Tyr-68, Asp-69, and Arg-71) failed to block assembly or eliminate function. Only the Arg-71 --> Ala substitution causes a substantial decrease in holoenzyme thermal stability. Tyr-68 --> Ala and Asp-69 --> Ala enzymes have lower K(m)(CO2) values, but these improvements are offset by decreases in carboxylation V(max) values. The Arg-71 --> Ala enzyme has a decreased carboxylation V(max) and increased K(m)(CO2) and K(m)(O2) values, which account for an observed 8% decrease in CO2/O2 specificity. Despite the fact that Arg-71 is more than 20 A from the large-subunit active site, it is apparent that the small-subunit betaA-betaB loop region can influence catalytic efficiency and CO2/O2 specificity.

RbcS suppressor mutations improve the thermal stability and CO2/O2 specificity of rbcL- mutant ribulose-1,5-bisphosphate carboxylase/oxygenase.

In the green alga Chlamydomonas reinhardtii, a Leu(290)-to-Phe (L290F) substitution in the large subunit of ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco), which is coded by the chloroplast rbcL gene, was previously found to be suppressed by second-site Ala(222)-to-Thr and Val(262)-to-Leu substitutions. These substitutions complement the photosynthesis deficiency of the L290F mutant by restoring the decreased thermal stability, catalytic efficiency, and CO(2)/O(2) specificity of the mutant enzyme back to wild-type values. Because residues 222, 262, and 290 interact with the loop between beta strands A and B of the Rubisco small subunit, which is coded by RbcS1 and RbcS2 nuclear genes, it seemed possible that substitutions in this loop might also suppress L290F. A mutation in a nuclear gene, Rbc-1, was previously found to suppress the biochemical defects of the L290F enzyme at a posttranslational step, but the nature of this gene and its product remains unknown. In the present study, three nuclear-gene suppressors were found to be linked to each other but not to the Rbc-1 locus. DNA sequencing revealed that the RbcS2 genes of these suppressor strains have mutations that cause either Asn(54)-to-Ser or Ala(57)-to-Val substitutions in the small-subunit betaA/betaB loop. When present in otherwise wild-type cells, with or without the resident RbcS1 gene, the mutant small subunits improve the thermal stability of wild-type Rubisco. These results indicate that the betaA/betaB loop, which is unique to eukaryotic Rubisco, contributes to holoenzyme thermal stability, catalytic efficiency, and CO(2)/O(2) specificity. The small subunit may be a fruitful target for engineering improved Rubisco.

Activase region on chloroplast ribulose-1,5-bisphosphate carboxylase/oxygenase. Nonconservative substitution in the large subunit alters species specificity of protein interaction.

In the active form of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC ), a carbamate at lysine 201 binds Mg(2+), which then interacts with the carboxylation transition state. Rubisco activase facilitates this spontaneous carbamylation/metal-binding process by removing phosphorylated inhibitors from the Rubisco active site. Activase from Solanaceae plants (e.g. tobacco) fails to activate Rubisco from non-Solanaceae plants (e.g. spinach and Chlamydomonas reinhardtii), and non-Solanaceae activase fails to activate Solanaceae Rubisco. Directed mutagenesis and chloroplast transformation previously showed that a proline 89 to arginine substitution on the surface of the large subunit of Chlamydomonas Rubisco switched its specificity from non-Solanaceae to Solanaceae activase activation. To define the size and function of this putative activase binding region, substitutions were created at positions flanking residue 89. As in the past, these substitutions changed the identities of Chlamydomonas residues to those of tobacco. Whereas an aspartate 86 to arginine substitution had little effect, aspartate 94 to lysine Rubisco was only partially activated by spinach activase but now fully activated by tobacco activase. In an attempt to eliminate the activase/Rubisco interaction, proline 89 was changed to alanine, which is not present in either non-Solanaceae or Solanaceae Rubisco. This substitution also caused reversal of activase specificity, indicating that amino acid identity alone does not determine the specificity of the interaction.

Suppressor mutations in the chloroplast-encoded large subunit improve the thermal stability of wild-type ribulose-1,5-bisphosphate carboxylase/oxygenase.

A temperature-conditional, photosynthesis-deficient mutant of the green alga Chlamydomonas reinhardtii, previously recovered by genetic screening, results from a leucine 290 to phenylalanine (L290F) substitution in the chloroplast-encoded large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC ). Rubisco purified from mutant cells grown at 25 degrees C has a reduction in CO(2)/O(2) specificity and is inactivated at lower temperatures than those that inactivate the wild-type enzyme. Second-site alanine 222 to threonine (A222T) or valine 262 to leucine (V262L) substitutions were previously isolated via genetic selection for photosynthetic ability at the 35 degrees C restrictive temperature. These intragenic suppressors improve the CO(2)/O(2) specificity and thermal stability of L290F Rubisco in vivo and in vitro. In the present study, directed mutagenesis and chloroplast transformation were used to create the A222T and V262L substitutions in an otherwise wild-type enzyme. Although neither substitution improves the CO(2)/O(2) specificity above the wild-type value, both improve the thermal stability of wild-type Rubisco in vitro. Based on the x-ray crystal structure of spinach Rubisco, large subunit residues 222, 262, and 290 are far from the active site. They surround a loop of residues in the nuclear-encoded small subunit. Interactions at this subunit interface may substantially contribute to the thermal stability of the Rubisco holoenzyme.

C172S substitution in the chloroplast-encoded large subunit affects stability and stress-induced turnover of ribulose-1,5-bisphosphate carboxylase/oxygenase.

Previous work has indicated that the turnover of chloroplast ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1. 39) may be controlled by the redox state of certain cysteine residues. To test this hypothesis, directed mutagenesis and chloroplast transformation were employed to create a C172S substitution in the Rubisco large subunit of the green alga Chlamydomonas reinhardtii. The C172S mutant strain was not substantially different from the wild type with respect to growth rate, and the purified mutant enzyme had a normal circular dichroism spectrum. However, the mutant enzyme was inactivated faster than the wild-type enzyme at 40 and 50 degrees C. In contrast, C172S mutant Rubisco was more resistant to sodium arsenite, which reacts with vicinal dithiols. The effect of arsenite may be directed to the cysteine 172/192 pair that is present in the wild-type enzyme, but absent in the mutant enzyme. The mutant enzyme was also more resistant to proteinase K in vitro at low redox potential. Furthermore, oxidative (hydrogen peroxide) or osmotic (mannitol) stress-induced degradation of Rubisco in vivo was delayed in C172S mutant cells relative to wild-type cells. Thus, cysteine residues could play a role in regulating the degradation of Rubisco under in vivo stress conditions.

Preliminary X-ray crystallographic study of wild-type and mutant ribulose-1,5-bisphosphate carboxylase/oxygenase from Chlamydomonas reinhardtii.

Ribulose-1,5-bisphosphate carboxylase/oxygenase is the key enzyme for photosynthesis. The wild-type and mutant (amino-acid substitutions in the catalytically important loop 6 region) enzymes from Chlamydomonas reinhardtii, a unicellular green alga, were crystallized. Wild-type, single-mutant (V331A) and two double-mutant (V331A/T342I and V331A/G344S) proteins were activated with cofactors CO2 and Mg2+, complexed with the substrate analog 2'-carboxyarabinitol-1,5-bisphosphate, and crystallized in apparently isomorphous forms. Unit-cell determinations have been completed for three of the enzymes. They display orthorhombic symmetry with similar cell parameters: wild type a = 130.4, b = 203. 3, c = 208.5 A; single mutant (V331A) a = 128.0, b = 203.0, c = 207. 0A; and double mutant (V331A/T342I) a = 130.0, b = 202.1, c = 209.7 A. Crystals of the wild-type and single-mutant (V331A) enzymes diffracted to approximately 2.8 A. A small crystal of the double-mutant (V331A/T342I) enzyme diffracted to approximately 6 A. A partial data set (68% complete) of the wild-type protein has been collected at room temperature to about 3.5 A.

Specificity for activase is changed by a Pro-89 to Arg substitution in the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase.

Tobacco activase does not markedly facilitate the activation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1. 39) from non-Solanaceae species, including the green alga Chlamydomonas reinhardtii. To examine the basis of this specificity, we focused on two exposed residues in the large subunit of Rubisco that are unique to the Solanaceae proteins. By employing in vitro mutagenesis and chloroplast transformation, P89R and K356Q substitutions were separately made in the Chlamydomonas enzyme to change these residues to those present in tobacco. Both mutants were indistinguishable from the wild type when grown with minimal medium in the light and contained wild-type levels of holoenzyme. Purified Rubisco was assessed for facilitated activation by spinach and tobacco activase. Both wild-type and K356Q Rubisco were similar in that spinach activase was much more effective than tobacco activase. In contrast, P89R Rubisco was not activated by spinach activase but was well activated by tobacco activase. Thus, the relative specificities of the spinach and tobacco activases for Chlamydomonas Rubisco were switched by changing a single residue at position 89. This result provides evidence for a site on the Rubisco holoenzyme that interacts directly with Rubisco activase.

Complementing substitutions at the bottom of the barrel influence catalysis and stability of ribulose-bisphosphate carboxylase/oxygenase.

The temperature-conditional photosynthesis-deficient mutant 68-4PP of Chlamydomonas reinhardtii results from a Leu-290 to Phe substitution in the chloroplast-encoded large subunit of ribulose-1, 5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39). Although this substitution occurs relatively far from the active site, the mutant enzyme has a reduced ratio of carboxylation to oxygenation in addition to reduced thermal stability in vivo and in vitro. In an attempt to understand the role of this region in catalysis, photosynthesis-competent revertants were selected. Two revertants, named R96-4C and R96-8E, were found to arise from second-site mutations that cause V262L and A222T substitutions, respectively. These intragenic suppressor mutations increase the CO2/O2 specificity and carboxylation Vmax back to wild-type values. Based on the crystal structure of the spinach holoenzyme, Leu-290 is not in van der Waals contact with either Val-262 or Ala-222. However, all three residues are located at the bottom of the alpha/beta-barrel active site and may interact with residues of the nuclear encoded small subunits. It appears that amino acid residues at the interface of large and small subunits can influence both stability and catalysis.

Elimination of the Chlamydomonas gene family that encodes the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase.

Ribulose-1,5-bisphosphate carboxylase/ oxygenase (EC 4.1.1.39) is the key photosynthetic enzyme that catalyzes the first step of CO2 fixation. The chloroplast-localized holoenzyme of plants and green algae contains eight nuclear-encoded small subunits and eight chloroplast-encoded large subunits. Although much has been learned about the enzyme active site that resides within each large subunit, it has been difficult to assess the role of eukaryotic small subunits in holoenzyme function and expression. Small subunits are coded by a family of genes, precluding genetic screening or nuclear transformation approaches for the recovery of small-subunit mutants. In this study, the two small-subunit mutants. In this study, the two small-subunit genes of the green alga Chlamydomonas reinhardtii were eliminated during random insertional mutagenesis. The photosynthesis-deficient deletion mutant can be complemented with either of the two wild-type small-subunit genes or with a chimeric gene that contains features of both. Thus, either small subunit is sufficient for holoenzyme assembly and function. In the absence of small subunits, expression of chloroplast-encoded large subunits appears to be inhibited at the level of translation.

Directed mutagenesis of chloroplast ribulose-1,5-bisphosphate carboxylase/oxygenase. Loop 6 substitutions complement for structural stability but decrease catalytic efficiency.

The structure of active-site loop 6 plays a role in determining the CO2/O2 specificity of chloroplast ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39). Rubisco from the green alga Chlamydomonas reinhardtii differs from higher plant Rubisco within the loop 6 region, and the C. reinhardtii enzyme has a CO2/O2 specificity 25% lower than that of higher plant enzymes. To examine whether differences in sequence may account for differences in catalytic efficiency, we focused on a conserved pair of residues that are in van der Waals contact at the base of loop 6. C. reinhardtii Rubisco contains Leu-326 and Met-349, whereas higher plant enzymes contain Ile-326 and Leu-349. By employing in vitro mutagenesis and chloroplast transformation, L326I and M349L substitutions were created within the Rubisco large subunit of C. reinhardtii. M349L had little effect, but L326I destabilized the holoenzyme in vivo and in vitro. When present together, the M349L substitution partially alleviated the instability resulting from the L326I substitution, but caused a 21% decrease in CO2/O2 specificity and a 74% decrease in the Vmax of carboxylation. Interactions between loop 6 and other structural regions are likely to be responsible for both holoenzyme stability and catalytic efficiency in higher plant Rubisco enzymes.

Pseudoreversion substitution at large-subunit residue 54 influences the CO2/O2 specificity of chloroplast ribulose-bisphosphate carboxylase/oxygenase.

Chlamydomonas reinhardtii mutant 31-4E lacks ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) holoenzyme due to a mutation in the chloroplast rbcL gene. This mutation causes a glycine54-to-aspartate substitution within the N-terminal domain of the Rubisco large subunit. In the present study, photosynthesis-competent revertants were selected to determine whether other amino acid substitutions might complement the primary defect. Revertants were found to arise from only true reversion or either of two forms of pseudoreversion affecting residue 54. One pseudorevertant has a glycine54-to-alanine substitution that decreases the accumulation of holoenzyme, but the purified Rubisco has near-normal kinetic properties. The other pseudorevertant has a glycine54-to-valine substitution that causes an even greater decrease in holoenzyme accumulation. Rubisco purified from this strain was found to have an 83% decrease in the Vmax of carboxylation and an 18% decrease in the CO2/O2 specificity factor. These results indicate that small increases in the size of amino acid side chains can influence Rubisco assembly or stability. Even though such changes occur far from the active site, they also play a significant role in determining Rubisco catalytic efficiency.

Nuclear Mutation Inhibits Expression of the Chloroplast Gene That Encodes the Large Subunit of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase.

Chlamydomonas reinhardtii mutant 76-5EN was recovered as a light-sensitive, acetate-requiring strain that failed to complement a chloroplast structural gene mutant of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39). Further genetic analysis revealed that the new mutation was inherited in a mendelian pattern, indicating that it resides within the nucleus. The 76-5EN mutant lacks Rubisco holoenzyme but has wild-type levels of whole-chain electron transport activity and chlorophyll. During a 1-min pulse labeling with 35SO42-, little or no Rubisco large-subunit synthesis occurred in the mutant. Nuclear-encoded small subunits were synthesized to a normal level and were subsequently degraded. When analyzed by northern hybridization, the 76-5EN mutant was found to have a decreased level of large-subunit mRNA. Large-subunit mRNA synthesis also appeared to be reduced during a 10-min pulse labeling with [32P]orthophosphate, but the labeled mRNA was stable during a 1-h chase. These results indicate that a nuclear gene mutation specifically disrupts the accumulation of large-subunit mRNA within the chloroplast. A deeper understanding of the nature of the 76-5EN gene may be useful for manipulating the expression of the agronomically important Rubisco enzyme.

Complementing substitutions within loop regions 2 and 3 of the alpha/beta-barrel active site influence the CO2/O2 specificity of chloroplast ribulose-1,5-bisphosphate carboxylase/oxygenase.

An acetate-requiring mutant of the green alga Chlamydomonas reinhardtii, named 28-7J, has been recovered using chemical mutagenesis. It lacks ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) holoenzyme, and accumulates only a small amount of the chloroplast-encoded large subunit. Pulse/chase experiments revealed that large subunits and nuclear-encoded small subunits are synthesized at normal rates. Because the mutant strain displayed uniparental inheritance and failed to complement a known chloroplast rbcL gene mutant strain, the 28-7J rbcL gene was cloned and sequenced to identify the new mutation. A single base change was found that causes large-subunit arginine-217 to be replaced by serine. This substitution occurs within alpha-helix 2 of the alpha/beta-barrel active site. When photosynthesis-competent revertants were selected from mutant 28-7J, revertant R14-A was found to contain a second mutation within the rbcL gene. This intragenic suppressor mutation, named S14-A, causes alanine-242 to be replaced by valine within beta-strand 3. Holoenzyme from the R14-A double-mutant strain was found to have a 51% reduction in the CO2/O2 specificity factor, primarily due to a 91% decrease in the Vmax of carboxylation. The Km for ribulose 1,5-bisphosphate was increased 2-fold. Although the mutant substitutions are separated by 24 residues within the primary structure, they are close to each other in the tertiary structure. In fact, the substituted residues are also close to lysine-201, which must be carbamylated and coordinated with Mg2+ to activate the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Directed mutagenesis of chloroplast ribulosebisphosphate carboxylase/oxygenase. Substitutions at large subunit asparagine 123 and serine 379 decrease CO2/O2 specificity.

Chloroplast-encoded large subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) are insoluble when separated from the holoenzyme or expressed in Escherichia coli, limiting directed mutagenesis to prokaryotic enzymes. In the present study, we performed directed mutagenesis and chloroplast transformation with the large subunit gene of Chlamydomonas reinhardtii. Two separate mutations were created that are known to influence the CO2/O2 specificity of prokaryotic enzymes. The asparagine 123 to glycine and serine 379 to alanine substitutions gave rise to photosynthesis-deficient mutants that synthesize normal levels of holoenzyme. The Vmax for carboxylation was reduced more than 95% and the Km(CO2) was increased more than 3-fold for both mutant enzymes. Km (O2) was slightly reduced for the glycine 123 enzyme, but increased more than 5-fold for the alanine 379 enzyme. CO2/O2 specificity factors for both enzymes are decreased by more than 70%. Km values for ribulose 1,5-bisphosphate are not significantly affected, but binding affinities for the transition-state analog 2-carboxy-D-arabinitol 1,5-bisphosphate are reduced. The changes brought about by these substitutions in the eukaryotic large subunit are different from the changes observed in prokaryotic enzymes.

Thermal Instability of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase from a Temperature-Conditional Chloroplast Mutant of Chlamydomonas reinhardtii.

Mutant 68-4PP of Chlamydomonas reinhardtii has only 10% of the normal level of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) holoenzyme when grown at 35[deg]C. However, when grown at 25[deg]C, the amount of holoenzyme is greater than 35% of the wild-type level, and the purified enzyme has a reduced CO2/O2 specificity factor. These mutant characteristics result from a chloroplast mutation that causes leucine-290 to be replaced by phenylalanine within the Rubisco large-subunit protein. A nuclear mutation (named S52-2B) was previously identified that can suppress both the in vivo instability and reduced CO2/O2 specificity of the mutant enzyme. However, the effect of this nuclear mutation on the in vitro stability of the holoenzyme was not resolved. In the present study, purified Rubisco from mutant 68-4PP was found to be less thermally stable than the wild-type enzyme, and it had maximal carboxylase activity at a lower temperature. When incubated at 35[deg]C, the mutant enzyme lost carboxylase activity at a much faster rate than the wild-type enzyme. However, the nuclear S52-2B suppresor mutation improved the thermal stability of the mutant enzyme in all cases. These results indicate that structural changes in mutant 68-4PP Rubisco can account for its observed inactivation in vitro and degradation in vivo. Such structural alterations are alleviated by the function of a nuclear gene.

Chloroplast heteroplasmicity is stabilized by an amber-suppressor tryptophan tRNA(CUA).

Photosynthesis-deficient mutants of the green alga Chlamydomonas reinhardtii were previously shown to arise from nonsense mutations within the chloroplast rbcL gene, which encodes the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39). Photosynthesis-competent revertants of these nonsense mutants have often been found to be stably heteroplasmic, giving rise to both mutant and revertant cells during mitotic or meiotic divisions under nonselective growth conditions. It was proposed that the stable heteroplasmicity might arise from a balanced polymorphism of suppressor and wild-type alleles of a tRNA gene maintained within the polyploid chloroplast genome. In the present study, we have focused on determining the molecular basis for the heteroplasmicity of one such revertant, named R13-3C, which was recovered from the 18-7G amber (UAG) mutant. Restriction-enzyme analysis and DNA sequencing showed that the amber mutation is still present in the rbcL gene of the revertant strain. In contrast, DNA sequencing of the suspected tRNA(Trp) gene of the revertant revealed a mutation that would change its CCA anticodon to amber-specific CUA. This mutation was found to be heteroplasmic, being present in only 70% of the tRNA(Trp) gene copies. Under nonselective conditions, the suppressor mutation was lost from cells that also lost the revertant phenotype. We conclude that stable heteroplasmicity can arise as a balanced polymorphism of organellar alleles. This observation suggests that additional tRNA suppressors may be identified due to their heteroplasmic nature within polyploid genomes.